Problem: Which of these numbers is prime? ${23,\ 33,\ 76,\ 91,\ 95}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 23 are 1 and 23. The factors of 33 are 1, 3, 11, and 33. The factors of 76 are 1, 2, 4, 19, 38, and 76. The factors of 91 are 1, 7, 13, and 91. The factors of 95 are 1, 5, 19, and 95. Thus, 23 is a prime number.